Mail::Box

Many of the e-mail modules use MIME::Parser, which reads
e-mails from files into Mail::Internet objects. Each
such Mail::Internet object is a single simplified message,
and you need MIME::Entity (which extends
Mail::Internet) to have access to message-parts, decoding,
and basic actions as replying.

One of the more developed modules which can handle sets of such messages
is the Mail::Folder. It is able to handle various
kinds of mail folders. As you can see, differences are purely stored
in the folder objects, and not stored in the messages, because each
type of folder only uses MIME::Entitys.